1. Field of the Invention
This invention relates to current sensing circuits for sensing alternating or pulsed current drawn from a power supply of an electronic instrument. More particularly, the invention relates to a sensing circuit that senses such current across a galvanic barrier between the power supply and the internal circuitry of the instrument.
2. Description of the Prior Art
Current sensing circuits are often found in electronic instruments to monitor the current the instrument is drawing from the power supply. The sensing circuit generates as an output a current sensing signal that is proportional to the total current drawn and transmits that signal to feedback circuitry, which adjusts the power supply accordingly to maintain a constant voltage level at the supply output.
In electronic instruments that use AC power lines as a power source, the power supply connected to the AC lines is required to be "external," or galvanically isolated from the internal instrument circuitry. This isolation ensures that an instrument operator is not exposed to electrical shock that could occur from uncontrolled power surges into the instrument through the AC lines. The current sensing circuit, too, must be galvanically isolated from the power supply so that no direct electrical connection exists across it between the external power supply and internal instrument circuitry. A well-known means for providing this isolation in a current sensing circuit is an isolation transformer with a primary-to-secondary windings ratio to produce a secondary current that is only a small fraction of the primary current in the external power supply. This secondary current is the source of the current sensing signal, be it the secondary current itself or a signal corresponding to that current. Rather than using an electrical connection, the isolation transformer uses magnetic flux linkage to transfer current across the galvanic barrier and thereby signal the feedback loop.
The accuracy of such a transformer in producing the current sensing signal is determined by the transformer's frequency response, defined as the range of frequency in which the transformer accurately transforms the primary current into the secondary current. This frequency response is a function of a number of variables, including the voltage amplitude on the secondary winding and the transformer size. Increasing the secondary voltage amplitude decreases the accuracy and narrows the frequency range by raising its low frequency boundary proportionally. On the other hand, increasing the transformer size increases the inductance and widens its frequency range by lowering the low frequency boundary.
Prior current sensing circuits have employed an isolation transformer with a resistor across the secondary windings to produce a current sensing signal. This signal, in effect the secondary voltage, increases as the secondary current increases, lowering the transformer accuracy by raising the lower frequency boundary. At primary current frequencies which fall below the low frequency boundary, the secondary current will not accurately reflect the input current and the sensing signal will contain substantial error. To compensate, prior design has enlarged the isolation transformer to increase its inductance. However, enlarging the transformer is undesirable where size and weight are critical criteria, such as on an etched circuit board.
A further drawback of prior circuits is loading of the transformer by the impedance of the internal circuitry, which affects the secondary voltage across a sense resistor.